1. Field of the Invention
The present invention relates to magnetic detection devices, and in particular relates to a device that detects a direction of a magnetic field changing with time by rotation etc., for example, one that detects a rotation speed and rotation angle of a rotating body.
2. Description of the Related Art
A conventional technology is disclosed in, for example, Japanese Laid-open Patent Publication No. H11-108689. The device that detects rotation of a rotating body is configured in such a way that a magnetized rotor that is magnetized to N and S poles alternately arranged along the outer circumferential surface thereof is rotated with the rotating body, and the number of changes in magnetic field caused by the rotation is detected. The magnetized rotor is made rotatable circumferentially by a rotor shaft. For example, the magnetized rotor shaft is assembled to an engine crank shaft or an axle so as to rotate with the crank shaft or the axle in an integrated manner.
It is also described there to use a magnetoresistive element as a detection element and detect changes in magnetic flux passing through the detection element when the magnetized rotor rotates. The magnetoresistive element shown in Japanese Laid-open Patent Publication No. H11-108689 is a tunneling junction magnetoresistive element (TMR element). As shown in FIG. 31, the TMR element includes a fixed layer 6 made of a ferromagnetic material whose magnetization direction is fixed, a non-magnetic middle layer 8 made of a non-magnetic material and a free layer 7 made of a ferromagnetic material whose magnetization direction can be freely changed by an external magnetic field, and is enabled to detect changes in magnetoresistance based on changes in the value of a current flowing across the layers depending on a magnetic field at a position where the element is arranged.
FIG. 31 shows variations in resistance value when an external magnetic field is applied that is parallel to the magnetization direction of the fixed layer 6 of the TMR element, which is the magnetoresistive element. When a parallel magnetic field greater than a certain value with respect to the magnetization direction of the fixed layer 6 is applied, the resistance value of the magnetoresistive element varies toward its minimum value and is then saturated. Whereas, when an antiparallel magnetic field greater than a certain value is applied, the resistance value of the magnetoresistive element varies toward its maximum value and is then saturated. FIG. 32 shows how the resistance value varies when a magnetic field saturating the magnetoresistive element shown in FIG. 31 is applied to the element and an angle formed with respect to the magnetization direction of the fixed layer 6 is changed. As shown in the figure, the resistance value varies with a period of 360 degrees, depending on the relative angle formed between the magnetization direction of the fixed layer 6 and the applied magnetic field direction (to be the magnetization direction of the free layer 7).
FIG. 33 shows a schematic configuration view of a conventional magnetic detection device equipped with a magnetized rotor and magnetoresistive elements. The reference numerals 1 and 2 denote magnetoresistive elements; the reference numeral 3, a magnetized rotor with a plurality of N and S poles alternately arranged along the outer circumferential surface thereof; the reference numeral 4, a rotation shaft of the magnetized rotor 3; and the reference numeral 5, a circuit substrate mounted with the magnetoresistive elements 1 and 2 or an IC including a processing circuit. A coordinate system is defined as follows: a direction of fixed layers of the magnetoresistive elements 1 and 2 are the y-axis; a direction perpendicular to the fixed layers, the z-axis; and a direction perpendicular to the y-z-plane, the x-axis. The magnetized rotor 3 is disposed in the proximity of the magnetoresistive elements 1 and 2, in the y-axis direction viewed from the magnetoresistive elements 1 and 2. The rotation shaft 4 of the magnetized rotor 3 is made nearly parallel to the z-axis. The magnetoresistive elements 1 and 2 are arranged apart from each other by a certain distance (Le) in the x-axis directions, and the processing circuit shown in FIG. 34 performs waveform shaping.
FIG. 35 shows variations in the resistance values of the magnetoresistive elements 1 and 2 and also a differential output voltage Vc and final output Vo processed by the circuit shown in FIG. 34 when the magnetized rotor 3 is rotated. Since the magnetoresistive elements 1 and 2 are arranged apart from each other by the distance Le in a rotation direction of the magnetized rotor 3 (x-axis direction), the variations in the resistance values of both elements are out of phase with each other by Le. When a magnetic pole pitch P of the magnetized rotor 3 is nearly the same as the distance Le between the magnetoresistive elements 1 and 2, the foregoing differential output voltage becomes nearly sinusoidal.
However, when the magnetic pole pitch P of the magnetized rotor 3 becomes greater than the distance Le between the magnetoresistive elements 1 and 2 as shown in FIG. 36, a magnetic field in the same direction is applied to the magnetoresistive elements 1 and 2, thereby creating periods (A) during which the differential output voltage becomes a constant voltage. The differential output voltage comes close to Vref in these periods as described above; therefore erroneous pulses easily occur in the final output due to disturbance noise. Moreover, since variations in the differential output voltage become gentle, positions of signal rising and falling edges of the final output easily shift, so that detection accuracy would be deteriorated.
In order to avoid the foregoing situation, it is effective to set the distance Le between the magnetoresistive elements 1 and 2 in accordance with the magnetic pole pitch P of the magnetized rotor 3; however, the greater the magnetic pole pitch P of the magnetized rotor 3 is, the longer the distance between the magnetoresistive elements 1 and 2 becomes, thereby increasing the size of the circuit board 5, which will resultantly push up the magnetic detection device costs. In addition, the distance between the magnetoresistive elements 1 and 2 needs to be adjusted for each magnetized rotor.
Furthermore, Japanese Laid-open Patent Publication No. 2009-300143 discloses a magnetic detection device in which a magnetoresistive element is arranged on a substrate parallel to the outer circumferential surface of the magnetized rotor, in a position of the element facing the rotor. Japanese Laid-open Patent Publication No. 2009-300143 also describes the magnetic detection device as diminishing effects of the hysteresis characteristics of the magnetoresistive element by using a DC bias magnetic field. However, according to this magnetic detection device, the magnetoresistive element detects magnetic field components in the directions of the rotation shaft of the magnetized rotor and those in directions perpendicular to radial directions and to the rotation shaft of the magnetized rotor, out of components of the magnetic field the magnetized rotor generates. Therefore, output from the magnetoresistive element becomes nearly sinusoidal, so that the problem with the magnetized rotor pitch cannot be solved, the same as Japanese Laid-open Patent Publication No. H11-108689.